1. Field of the Invention
Aspects of the present invention relate to a composition for forming an electron emission source, an electron emission source formed from the composition and an electron emission device including the electron emission source. More particularly, aspects of the present invention relates to a composition for forming an electron emission source, which composition includes a polymer having a terminal group and/or main chain by which adhesion with a substrate, developing properties, photosensitivity, printability, current density, etc., can be increased. Aspects of the present invention further relate to an electron emission source formed from the composition and an electron emission device including the electron emission source. An electron emission source that has fine micro patterns and an excellent current density can be obtained using the composition for forming an electron emission source.
2. Description of the Related Art
Generally, electron emission devices use a hot cathode or a cold cathode as an electron emission source. Examples of electron emission devices using a cold cathode include a field emitter array (FEA) type, a surface conduction emitter (SCE) type, a metal insulator metal (MIM) type, a metal insulator semiconductor (MIS) type, and a ballistic electron surface emitting (BSE) type.
The FEA type of electron emission device utilizes the principle that when a material with a low work function or a high β-function is used as an electron emission source, electrons are easily emitted in a vacuum due to an electric field difference. FEA devices that include a tip structure primarily composed of Mo, Si, etc. and having a sharp end, and carbon-based materials such as graphite, diamond like carbon (DLC), etc., as electron emission sources have been developed. Recently, nanomaterials such as nanotubes and nanowires have been used as electron emission sources.
The SCE type of electron emission device is formed by interposing a conductive thin film between a first electrode and a second electrode, which are arranged on a substrate so as to face each other, and producing microcracks in the conductive thin film. When voltages are applied to the first and second electrodes and an electric current flows along the surface of the conductive thin film, electrons are emitted from the microcracks constituting the electron emission source.
The MIM type and the MIS type of electron emission device include a metal-insulator-metal structure and a metal-insulator-semiconductor structure, respectively, as an electron emission source. When voltages are applied to the two metals in the MIM type or to the metal and the semiconductor in the MIS type, electrons are emitted while migrating and accelerating from the metal or the semiconductor having a high electron potential to the metal having a low electron potential.
The BSE type of electron emission device utilizes the principle that when the size of a semiconductor is reduced to less than the mean free path of electrons in the semiconductor, electrons travel without scattering. An electron-supplying layer composed of a metal or a semiconductor is formed on an ohmic electrode, and then an insulator layer and a metal thin film are formed on the electron-supplying layer. When voltages are applied to the ohmic electrode and the metal thin film, electrons are emitted.
FEA type electron emission devices can be categorized as top gate types and under gate types according to the arrangement of the cathode and gate electrode and can be categorized as diodes, triodes, tetrodes, etc., according to the number of electrodes used.
Electron emission sources in the electron emission devices described above can be composed of carbon-based materials, such as, for example, carbon nanotubes. Carbon nanotubes have excellent conductivity and electric field focusing effects, small work functions, and excellent electric field emission characteristics, and thus can function at a low driving voltage and can be used for large displays. For these reasons, carbon nanotubes are considered as an ideal electron emission material for electron emission sources.
Methods of forming electron emission sources containing carbon nanotubes include, for example, a carbon nanotube growing method using chemical vapor deposition (CVD), etc., and a paste method using a composition that contains carbon nanotubes and a vehicle. When using the paste method, manufacturing costs decrease, and large-area electron emission sources can be obtained. Examples of a composition for forming electron emission sources that contains carbon nanotubes are disclosed, for example, in U.S. Pat. No. 6,436,221.
However, when an electron emission source is formed using a conventional paste method, the printability of the composition for forming the electron emission source is bad, and repeated printings are difficult. In addition, when the electron emission source is patterned after printing by creating cured and uncured parts of a conventional composition for forming an electron emission source and then removing the uncured part, it may be difficult to remove the uncured part because of poor solubility in the developer and poor developing properties. Moreover, the adhesion between the cured part and the substrate may be poor and thus, the cured part may also be removed.